Spatial light modulators (SLM's) are transducers that modulate incident light in a spatial pattern corresponding to an electrical or optical input. The incident light may be modulated in phase, intensity, polarization, or direction. The light modulation may be achieved by a variety of materials exhibiting various electro-optic or magneto-optic effects and by materials that modulate light by surface deformation. SLM's have found numerous applications in the areas of optical information processing, projection displays, and electrostatic printing.
Many SLM applications involve some sort of data storage on a per pixel basis. For example, when used for video display systems, SLM's take the place of raster-scan electron beam devices. The SLM is comprised of an area array of individually addressable pixel elements, each of which corresponds to a point of an image. For display, light from each pixel element is magnified and projected to a display screen by an optical system. The type of modulation depends on how the modulator is combined with an optical system. Typically, each pixel is addressed and data pertaining to its desired state is loaded into a memory cell associated with that pixel. Then, the pixels change state to produce an image.
A thorough discussion of various types of SLM's is set out in the background section of U.S. Pat. No. 4,956,619. Devices sharing the same structural features as SLM's have been used for applications that do not involve light modulation. Examples of such applications are optical switching, light beam steering, and acceleration measurement. However, the term "SLM" is nevertheless used herein to describe these devices because of they also are characterized by addressable, micro-mechanical elements.
A frequently used type of SLM is the deformable mirror device (DMD), in which each pixel element is a tiny mirror, each capable of separate mechanical movement in response to an electrical input. Incident light may be modulated in direction, phase, or amplitude by reflection from each pixel element. Various DMD architectures have been developed, which include variations with respect to the type of mirror elements and the addressing circuit. Mirror element types include elastomer, membrane, and cantilever or torsion beam types. Addressing may be by e-beam input, optically, or by means of an electronic memory circuit. Cantilever and torsion beam architectures, in combination with integrated circuit addressing, are described in a article entitled "Deformable-Mirror Spatial Light Modulators", by Larry J. Hornbeck, published in Proc. SPIE 1150, pp. 86-102 (1990). As described therein, ideally, the memory cell circuit is designed so that it may be integrated with a mirror element superstructure, using conventional integrated circuit techniques. U.S. Pat. No. 4,956,619 describes a method of fabricating such DMD's. For example, in projection display applications, each SLM pixel element is associated with one or more memory cells.
In the past, the memory cell circuits for SLM's have been custom designed and usually resemble conventional dynamic random access memories (DRAM's). A description of the structure and operation of one form of memory cell design for DMD's is described in U.S. Pat. No. 5,079,544 entitled "Standard Independent Digitized Video System", filed Feb. 27, 1989, and assigned to the same assignee as the present invention.
A problem with dynamic memory designs is that relatively large storage capacitors are required at the cell site. When used in an SLM integrated circuit, these capacitors are susceptible to interlayer dielectric effects. Also, bi-stable SLM pixel designs require both signal polarities at the pixel site. To accomplish this bi-polarity with DRAM memory cells, either two drive lines and associated storage capacitors are required, or an inverter must be added to each cell. Finally, applications for SLM's involve illumination of the SLM, and as illumination increases, the refresh requirements of DRAM cells due to the charge carriers generated by illumination increases.
Although the use of static random access memory (SRAM) cells for the memory cell circuit would overcome some of the limitations of DRAM-based designs, there are problems associated with providing the power required for the SRAM cells. In SLM devices that require higher voltage levels for operating the array elements, if the same voltage is used to power the SRAM circuit, the result is a large current draw during writing to the cell as well as reduced writing speed. A need exists for an SRAM-based SLM design that does not have these limitations.